1. Field of the Invention
The present invention relates to intermittent operation receivers such as pagers.
2. Description of the Prior Art
Recently, in FSK (Frequency Shift Keying) receivers such as pagers, direct conversion systems have begun to be put to practical use. In such systems the main portions of the receivers may be formed as an IC without requiring any expensive and large-sized individual parts such as high and intermediate frequency filters to achieve miniaturization and cost reduction (see Japanese Patent Publication JP-A-4-86139). There are systems which directly convert the received signals to a lower frequency signal (corresponding to a frequency deviation) to demodulate the desired signal by low frequency signal processing.
In such systems, mentioned above, however, the low frequency section produces a large amount of noise compared to heterodyne systems. In order to control such noise, an increase in the gain of the high frequency section is required. If no filters are used in the high frequency section, such system is susceptible to disturbance of reception due to possible intermodulation occurring when a large input signal is received.
In order to cope with this situation, provision of filters in the high frequency section is effective, which would be a hindrance to miniaturization and cost reduction of the receivers. Such arrangement cannot constitute a receiver which is capable of switching channels. Another effective measure is the use of an automatic gain control method which detects a received signal strength to control the gain of the high frequency section when a large signal is received.
However, since the conventional automatic gain control system is composed of a large feedback loop, it would take some time for control to shift from the detection of a received signal strength to the start of control of the gain of the high frequency section. Thus the use of such automatic gain control system in intermittent operation receivers such as pagers is difficult.